Co-Existence and niche differentiation of sulfur oxidizing bacteria in marine environments

Reduced sulfur compounds and sulfur-oxidizing prokaryotes (SOP) are widely distributed in the marine environment. Diverse microbial lineages thrive on the oxidation of reduced sulfur. They co-exist successfully by the adaptive radiation into different physiological and ecological niches. However, the factors determining this differentiation and SOP distribution are largely unknown. Environmental factors, like pH, temperature and salinity, as well as the physiological capabilities of different SOPs for sulfur-oxidation and carbon assimilation likely govern the niche-differentiation. Therefore, as part of multiple collaborative studies, I studied the influence of substrate quality and availability on structuring sulfur-oxidizing microbial communities in different marine habitats. First, the role of elemental sulfur (S0), in particular cyclooctasulfur (S8), as substrate for SOPs in marine benthic habitats was examined (Chapter II). We observed a specific association between Sulfurimonas/Sulfurovum-related Epsilonproteobacteria and S0/S8 regardless of the habitat. We propose that substrate quality effects SOP diversity and niche differentiation, and the capability to oxidize S8 probably provides a competitive advantage to the Sulfurimonas/Sulfurovum-group. Moreover, we investigated the diversity and distribution SOPs along gradients of a sulfide, oxygen and light in a highly sulfidic marine karst lake (Lake Rogoznica, Chapter III). The comprehensive analysis of microbial diversity revealed a community shift from phototrophic to chemotrophic sulfur oxidation during holomixis and tight coupling between sulfide and oxygen concentration and the sulfur-oxidizing microbial community in Lake Rogoznica. In two further studies, we explored different aspects of carbon assimilation in hydrothermally influenced habitats dominated by thiotrophic Sulfurimonas/Sulfurovum-related Epsilonproteobacteria. We demonstrated the effects of temperature and/or substrate flux on carbon-isotope fractionation during CO2 assimilation in environmental samples (Chapter IV). Furthermore, we showed that these and other hydrothermal vent associated thiotrophs do not incorporate acetate (Chapter V), despite their heterotrophic potential. Other microorganisms, not involved in oxidative sulfur cycling at hydrothermal vents, showed high activity and growth after the input of organic substrate. In summary, this thesis contributes to the general understanding of microbial ecology in sulfur-rich environments by provides novel insights into diversity and niche in sulfur-oxidizing microbial communities.